A main striving force in the development of wireless/cellular communication networks and systems is to provide, apart from many other aspects, increased coverage or support of higher data rate, or a combination of both. At the same time, the cost aspect of building and maintaining the system is of great importance and is expected to become even more so in the future. As data rates and/or communication distances are increased, the problem of increased battery consumption is another area of concern.
Until recently the main topology of wireless communication systems has been fairly unchanged, including the three existing generations of cellular networks. The topology of existing wireless communication systems is characterized by the cellular architecture with the fixed radio base stations and the mobile stations as the only transmitting and receiving entities in the networks typically involved in a communication session.
Several new transmission, or radio access, technologies have been proposed to increase capacity, flexibility and/or coverage in the communication systems. A promising technology is Orthogonal Frequency Domain Multiplexing (OFDM) that transmits multiple signals simultaneously over a wired or wireless communication medium. In wireless communications, the OFDM receiver is relative simple, since the multiple data streams are transmitted over a number of parallel flat fading channels. In fact, equalization is not done in the time domain; instead, one-tap filters in the frequency domain are sufficient. Despite this simplicity, uncoded OFDM transmission lacks inherent diversity that greatly helps to combat loss in the radio propagation environment, i.e. path loss, fast fading, etc.
One way to introduce diversity in the received signal is to utilize multiple antennas at the transmitter and possibly also at the receiver. The use of multiple antennas offers significant diversity and multiplexing gains relative to single antenna systems. A system utilizing multiple antennas both at the transmitter and at the receiver is often referred to as Multiple-Input Multiple-Output (MIMO) wireless systems. The spatial diversity offered by such systems can thus improve the link reliability and the spectral efficiency relative to Single-Input Single-Output (SISO) system.
An alternative approach to introduce macro-diversity is cooperative relaying. Cooperative relaying systems have many features and advantages in common with the more well-known multihop networks, wherein typically, in a wireless scenario, a communication involves a plurality of transmitting and receiving entities in a relaying configuration. Such systems offer possibilities of significantly reduced path loss between communicating (relay) entities, which may benefit the end-to-end (ETE) users. The cooperative relaying systems are typically limited to only two (or a few) hop relaying. A typical cooperative relaying system comprises of an access point, for example a radio base station which communicates with one or more user equipments, for example a mobile station, via a plurality of relay nodes.
In contrast to multihop networks, cooperative relaying systems exploits aspects of parallelism and also adopts themes from advanced antenna systems. These systems have cooperation among multiple stations or relay nodes, as a common denominator. In recent research literature, several names are in use, such as cooperative diversity, cooperative coding, and virtual antenna arrays. In the present application the terms “cooperative relaying system” and “cooperative schemes/methods” is meant to encompass all systems and networks utilizing cooperation among multiple stations and the schemes/methods used in these systems, respectively. The term “relaying system” is meant to encompass all systems and networks utilizing relying in any form, for example multihop system and cooperative relaying systems. A comprehensive overview of cooperative communication schemes are given in Cooperative Diversity in Wireless Networks: Algorithms and Architectures, J. N. Laneman, Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., August 2002.
Various formats of a relayed signal may be deployed. A signal may be decoded, re-modulated and forwarded, or alternatively simply amplified and forwarded. The former is known as decode-and-forward or regenerative relaying, whereas the latter is known as amplify-and-forward, or non-regenerative relaying. Both regenerative and non-regenerative relaying is well known, e.g. by traditional multihopping and repeater solutions respectively. Various aspects of the two approaches are addressed in “An Efficient Protocol for Realizing Distributed Spatial Diversity in Wireless Ad-Hoc Networks”, J. N. Laneman and G. W. Wornell, Proc. of ARL FedLab Symposium on Advanced Telecommunications and Information Distribution (ATIRP-2001), (College Park, Md.), March 2001.
Diversity gain is particularly attractive since it offers increased robustness of communication performance as well as allowing reduction of experienced average SNR for the same BER. In addition the cooperative relaying may provide other positive effects such as beamforming (or directivity) gain, and spatial multiplexing gain. The general benefits of the mentioned gains include higher data rates, reduced outage primarily due to different forms of diversity, increased battery life, and extended coverage.
There are several schemes that offer diversity gain: Alamouti diversity based cooperative relaying for example described in “Distributed Space-Time Coding in Cooperative Networks”, P. A. Anghel et al, Proc. of the Nordic Signal Processing Symp., Norway, October 2002. coherent combining based relaying, which in addition offer a beamforming gain as described in “Large-Scale Cooperative Relay Network with Optimal Coherent Combining under Aggregate Relay Power Constraints”, P. Larsson, Proc. Future Telecommunications Conference (FTC2003), Beijing, China, 9-10/12 2003. pp 166-170. and relay cyclic delay diversity as described in WO06121381. According to the scheme the relay nodes, in their forwarding between the base station and the user equipment, applies cyclic shifts to their respective forwarded OFDM symbols.
These schemes require two transmission phases for each down link (DL) and up link (UL) direction: for example in the DL, in the first transmission phase the basestation transmits to the relay node, and in the second transmission phase the relay node transmits to the user equipment. The two phase transmission methods may effectively reduce the data throughput by half.